To take an extreme example, look at the shirt below. (Graciously and garishly provided by the model as an example of a bad wardrobe choice)
No lighting in the world is going to help here. The strong pattern and extreme colors overwhelm the image. More often than not, bold patterns are going to make it difficult to read the shape of the belly. From the front, the shape of the belly is totally lost.
From the side, the situation is improved, but the focus is still drawn away from the subject.
A finer or more subtle pattern may work. A bold pattern can also work if it helps to define the subject as below.
A pattern works best when it creates visual interest. In the case of a maternity portrait, it should help to define the shape of the belly.
Another thing to be aware is support, and I don't mean the emotional kind. This can be an issue for maternity photos, as the mom-to-be may not want her bra to show, but also may not be happy with the results without it.
Something else to avoid where possible is flesh-tones. While there are many, many colors that can be considered "Flesh-tones", for the purpose of this guide a flesh-tone is the color of your subject's skin. The problem with flesh-tones is that they read ambiguously. The photo on the left, below, almost gives the impression of a nude, but not in a positive or interesting way. The photo would be better if it simply were photographed as a nude as the next two example show.
A more common wardrobe choice is the mom-to-be wearing her husband's shirt, open to show her belly. It is a long-standing classic of maternity photography, but most often it is done poorly. In the previous section on lighting I showed the wrinkled white shirt on a wrinkled white backdrop, which creates a lot of unaesthetic visual confusion. Even without the issue of wrinkles, the bulk of the larger shirt tends to be unflattering or, at least, uninteresting.
In order to do the "dad shirt" well, you need to provide contrast conceptually. One way to do this is to contrast the bulkiness of the shirt with the lines of the legs and the form of the belly.
This is a good place to apply a squint test. Squint your eyes at the two examples below...
Notice how the photo of her in the black dress is much clearer, even if stand back and squint your eyes. The dress is simple and modest, but drapes nicely, creating good lines which improve the photo's composition.
The issue is not simply one of contrast. The white on black photo has just as much contrast between clothing and background. It has more to do with line and form. Try squinting again...
A better outfit also brings with it more opportunity. Good line and form is essential for a good silhouette.
Another thing to consider with wardrobe is how well the style and color of the clothes suit the model.
The very bold yellow does not suit the model here and while the hood seems like a cute idea it doesn't feel right. (Perhaps because it makes her look like a cone-head.) Even in black and white, the color still has some effect, complementing or failing to compliment the skin.